Summary and Concluding Remarks

Detailed mapping of the Popigai impact structure indicates that faulting within impact rock sequences is widespread throughout the crater. Two hierarchical groups of fractures and faults are distinguished: (1) local faults and fissures within impact rock sequences and (2) main radial and concentric faults sets, which determine the block structure of the crater. The distinguishing of the proper post-impact deformation features from the patterns that belong to the regional tectonic framework on the one side and to the impact on the other side, is frequently ambiguous, because of the post-impact movements exploit syn-impact fault systems, whereas subsequent regional tectonism accentuate impact-related faults as well.

Local fractures are mainly detected from aerial photo interpretation. In some areas with thick sequences of lithic breccia, the density of faults and fractures is as much as 5-10 per 10 km2. Individual faults do not exceed 6 km in length, and some show a vertical or horizontal separation of up to 10 m; however, tear fractures without separation are predominant. There is no discernible preferred orientation of individual fractures, but fracture systems have near-radial or near-concentric directions, reflecting the subsurface structure of the crater.

These small-scale fractures are specific to the Popigai crater in their density and their geometry and are clearly post-consolidation of the impact melts and breccias. The intensity of impact-induced brittle deformations in the target decreases regularly outwards from the crater and attenuates at the distance of 70-75 km from the crater center (Masaitis et al. 1998). From the 1:200,000 geological mapping data, the Proterozoic and Paleozoic sedimentary rocks outside of this deformed zone are much lesser fractured, compared to impact rocks within the Popigai. Thus, the late modification-stage is most likely dating the formation of local fractures, which probably originated during the compaction of strongly inhomogeneous and unconsolidated thick impact sequences; these include lithic breccia and suevites with tagamite bodies of variable morphology. The thickness of these sequences reaches several hundred meters in the annular trough. Growth of stress along lithological boundaries during compaction caused a sudden ground subsidence as an ultimate strength of matter was exceeded (Rice 1980). The presence of long tear fractures in tagamites indicates that the fracturing occurred during the diagenesis of impact rocks when impact melt became solid to produce brittle deformations.

Post-impact block displacements were controlled predominantly by impact-related radial and concentric faults. The distribution of subsided and elevated

Fig. 15. Clastic dike, 0.8-1.2 m thick, within massive impact melt rock (tagamite). The dike forms a shallow vertical channel in the outcrop wall due to its lesser resistance to weathering compared to tagamite. The location is on the east of the Chordu-Daldyn River valley, 11 km from the estuary.

Fig. 16. Displacement of the Chordu-Daldyn riverbed during 17 years (from 1984 to 2001). The deflection of stream is caused by the neotectonic rise of the southern block of the Popigai structure (at left on the photograph).

blocks, as well as the restriction of an uplift of Pliocene-Early Quaternary sediments to the southern part of the crater and its southern surroundings, and repeated accumulation of sediments in linear areas during a continuous period, suggest that the principal contribution is a regional tectonic overprint (namely, the rise of the Anabar Shield during Neogene-Quaternary period; Novikov 1997), rather than relaxation movements of the post-impact block tectonics of the Popigai crater.

It is important to note that the post-impact tectonic activity in the Popigai has not ceased up until today. For instance, the thickness of recent alluvial deposits in the Popigai River valley is determined by neotectonics (Plotnikova 1990). The Popigai depression differs markedly from surrounding areas by the intense development of thermokarst; during the detailed prospecting of the crater in the last 20 to 30 years, some displacements of riverbeds, changes of shape of lakes, etc., were observed. For example, a displacement of the Chordu-Daldyn riverbed has occurred (Fig. 16). Because the Chordu-Daldyn valley corresponds to a large concentric fault zone within the Popigai crater, this phenomenon indicates a repeated rise of the southern block, which is adjacent to the Anabar Shield.

Thus, the 35-Ma-long post-impact modification history of the Popigai crater is determined by the superimposition of the regional tectonics on the long-term relaxation movements. As a whole, the late modification stage tectonics is found to have only an insignificant effect on the Popigai crater, so that both the original structure and the crater topography have been retained in good condition.